For various reasons I've been considering replacing my ECM (ECU) with a stock, unflashed one. I have heard that supposedly the ECM and Immobilizer devices are a married unit, so if you replace the ECM only, the car will not run. Can anyone confirm or refute this? I'd like to figure out for sure before I spend the money on another ECM.

I have a stock map. However... if you bring it in with a problem, they know you're ran a modified map, thereby voiding the warranty. I figured that out when piston #4 melted through no fault of the mapping I was running and Subaru only covered 50% of the replacement of the block.

Since the repairs, I've run it on the stock map, stock exhaust, etc. The only power mod I have is my AVO intercooler at this point. I've been having a bit of roughness at idle - no codes or anything - but am reluctant to take it in to the dealership with that same ECM in the car, because they could just as easily deny any warranty coverage again...

When swapping ECUs, you will have to reprogram the BIU and all your keys with the new ECU. This can only be done through a dealer, or if you know someone at a Subi dealer that can lend you the SSM after hours, and give you the correct codes to get into the ECU/BIU programming menus.

when reading the ecm with ssm it has its own odometer. and you can see the serial number..

so would could be done .. prior to modifying your car you get a new cluster, new body integrated unit and new ecm swap those out and keep them and then modify the ecm. then when you go to get waranty work done you put them back.

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Now that's thinking out of the boxer!
fyi all 05 + legacy's have built in code reader